1. Field of the Invention:
This invention pertains to the art of transport refrigeration units, and in particular to an airflow control arrangement for a unit provided with air shutters at its front and top to control the airflow through the refrigerant condenser and the radiator for the internal combustion engine of the unit.
2. Description of the Prior Art:
Transport refrigeration units of the type with which this invention is concerned are designed to provide as controlled temperature in the conditioned space, such as in a truck trailer or a railway car, which may be as low as -20.degree. F. (-29.degree. C.) or as high as 70.degree. F.(21.degree. C.), depending upon the particular food cargo to be transported. These temperatures are to be obtained even though the ambient temperature may be as high as that of the desert, or as low as that found in the north during severe cold waves. As a result, the transport refrigeration units are designed to be quite versatile in operation and provide heating and cooling at various capacity rates.
One arrangement which has been considered to be helpful with respect to a heating operation and wherein the heating capability is increased at low ambient temperatures is the provision of shutters in the openings in the external cabinet through which condenser and radiator cooling air is passed. To the best of our knowledge such shutters have heretofore comprised a slotted stationary plate and a slotted slidable plate operated automatically by one or more thermal motors which will move both shutters toward open or closed position simultaneously in response to given increasing and decreasing temperatures respectively, in the interior of the cabinet. Such an arrangement has functioned satisfactorily for some purposes, but there have also been problems with such an arrangement which this invention is intended to solve. As noted before the units are expected to function to produce some given interior temperature irrespective of significant variations in the ambient temperature. One difficulty which has been experienced with the prior shutter arrangement is that under some such conditions the compressor discharge pressures rise to a level so quickly after the unit has been started that the unit automatically shuts down. Thus, it has been experienced that with ambient temperatures even in a mid-range such as 55.degree. to 75.degree. F. (13.degree. to 24.degree. C.) and with the sliding shutters control set as say, 85.degree. F. (29.degree. C.), when a number of units are tobe started as in a railroad yard, some units will have the shutters open soon enough that the unit does not shut down while other units will reach an excessive discharge pressure and automatically shut down. At higher ambient temperatures the likelihood is that even more units will automatically shut down from the high discharge pressure. One solution to this latter problem is to have the shutters open at a lower set point temperature such as 75.degree. F. (24.degree. C.), and while this has been effective to avoid most shutdowns, it also means that when increased capacity for heating is desired it will be lost at an earlier time than is desired because of the earlier opening of the shutters.
The slotted plate shutter arrangement also created some problems in that it restricts airflow beyond that of an ordinary grille, an accordingly under conditions of very high ambient temperatures, and with a significant load on the unit, the airflow may not be adequate to avoid a compressor shutdown because of the high compressor discharge temperatures.
It is the aim of this invention to provide an airflow control arrangement with shutters in which the above-noted difficulties of the prior art are significantly reduced.